Method of preparing semi-conductor alloys



Aug. 4, 1959 I R. v. JENSEN 2,898,249

4METHOD oF PREPARING SEMI-CONDUCTOR ALLoYs original Filed June 10.l 1954 4 Il Y 2.

i L\ X U INVENTOR.

REBER-r V. JENSEN Irfan/lr 2,898,249 Patented Aug. 4, 1959 METHOD F PREPARING SEMI-CONDUCTOR ALLOYS Robert V. `lensen, Fairless Hills, Pa., assignor to Radio Corporation of America, a corporation of Delaware Continuation of application Serial No. 435,783,1une 10, lsl'lhis application January 31, 1958, Serial No.

Claims. (Cl. 14S-1.6)

This application is a continuation of my copending application Serial Number 435,783, filed lune l0, 1954, now abandoned, and entitled Semi-Conductor Alloys.

This invention relates, generally, to improved methods of manufacturing alloy materials. More particularly, the invention relates to improved methods of manufacturing alloys such as single crystal alloys of germanium and silicon.

One object of the present invention is to `provide an improved method of manufacturing alloy materials.

Another object of the invention is to provide an improved method of manufacturing single crystalline alloys of semi-conducting materials, such as germanium and silicon.

In the preparation of alloys according to the method of the present invention, an alloying agent is added to a base material, which may be for example, germanium or silicon, in successive but very gradual amounts so that the concentration of the alloying agent in the base material is considerably higher at the end of the manufacturing operation than it is at the beginning. The alloying agent is introduced gradually into the crystal lattice of the base material. Crystalline alloy materials can be prepared by passing a molten zone along a bar-shaped specimen of base material in the presence of successively larger amounts of alloying agent. lf a seed crystal is contacted to an end of the bar, or ingot, and that end of the bar melted and re-solidiiied rst, single crystals can be prepared. Single crystals of semi-conductor alloys can also be prepared by pulling a crystal from a melt to which is added an alloying agent during the course of the crystal-pulling operation.

When excess impurities or alloying agents are suddenly introduced into the crystal lattice of either germanium or silicon, the crystallization often yields a polycrystalline mass or a polyphase material characterized by separate crystals of alloying agent or impurity and base material. By effecting the gradual introduction of an alloying agent, it is possible to produce single crystalline semi-conductor alloy materials suitable for fabrication into semi-conductor devices.

These and other objects and advantages of the instant invention will be more fully described by reference to the accompanying drawing in which:

Figure 1 is a plan View of a boat type crucible charged with constituents for preparing an alloy according to one `embodiment of the present invention.

Figure 2 is an elevational cross-sectional View taken along the section line Il--II of Figure 1 of the charged crucible.

Figure 3 is a crosssectional elevational View similar to that of Figure 2 and illustrates a variation of the method of preparing a charge illustrated in Figure 2 which includes the apposition of wedge-shaped pieces of semiconductor base material and alloying agent.

Figure 4 is a cross-sectional elevational view of the crucible illustrated in Figure l being drawn through an induction type furnace.

Figure 5 is a partially cross-sectional elevational view of an apparatus for pulling a crystal from a melt to which an alloying agent is added while a crystal is being pulled.

Similar reference characters are applied to similar elements throughout the drawing.

In general, the process of the present invention com= prises preparing a charge of a semi-conductor base material, which may initially be either germanium, silicon, indium antimonide or other semi-conductor material, in a crucible and applying to this charge heat sufficient to melt the base material. When single crystals are desired, single crystal growth of the base material is begun by seeding. Alloying agent is added gradually to the molten base material and as the crystal grows it contains increasingly greater proportions of the allo-ying agent dispersed uniformly throughout the crystal lattice structure. When a desired alloying agent concentration is reached, crystal growth is continued at this level by adding to the melt appropriate amounts of alloying agent and base material so that a balance is maintained.

The solubility relation of the minor component to the major component is expressed as a ratio of the concentration of the minor component dissolved in the solid phase o f the major component to the concentration of the minor component dissolved in the liquid phase of the major component. This ratio is the distribution coecient. When different materials are prepared according to the present invention, differences in the rate of crystal growth and in distribution coeliicients require special adjustments of the conditions of crystal growth in each case.

Examples of processes in accordance with the present invention are as follows:

Example I In the system of Figures l and 2, the elongated boatlike crucible 6 is charged with a rod-shaped piece of base material 2, such as germanium, and a purified seed crystal 4 is placed at one end of the crucible 6 adjacent to the germanium charge. Depressions 8 are cut into the surface of the germanium charge, about .25 inch apart for example, over the greater part of its length. Into these depressions are inserted predetermined quantities of an alloying agent 10, such as indium. The indium may be in any convenient form, such as pellets weighing about 70 milligrams and the desired weight of alloying agent added to a depression may be obtained Iby using a plurality of pellets.

As shown in Figure 4, the crucible is placed within a tubular enclosure 11 and the crucible is gradually propelled, at about 2.5 inches per hour or less, for example, through a ringshaped induction heating element 12 starting at the part ol` the crucible adjacent to the seed crystal 4. The crucible is supported within the enclosure 11 by a ring-shaped member 5. At the start of the pulling operation, when the temperature reaches the melting point of the germanium, material adjacent to the seed melts, wets and melts a portion of the seed crystal and then begins to recrystallize as an extension of the crystal lattice of the seed as this part of the crucible leaves the heated zone i4. Successive segments of the charge are melted and recrystallized as the heated Zone which is at a temperature suicient to liquefy the charged material passes along the length of the crucible. Larger and larger amounts of alloying agent are incorporated in the growing crystal as the heated Zone passes along the length of the charge.

When indium is the alloying agent in germanium, its distribution coeliicient is .001 which shows that one thousand-times as much indium is dissolved in molten |germanium -as in crystalline germanium under these' growing conditions. Therefore as an alloy single crystal forms, the concentration of indium in the molten germanium decreases only very slowly as fresh germanium enters the molten zone. However, by adding indium slowly as germanium is added to the molten zone, it is possible to increase the concentration of indium in the molten zone and to saturate the single crystal lattice with indium atoms. This saturation level of indium in germanium reaches about one part in ten thousand.

Example Il Figure 3 illustrates a modification of the method described in Example I for the preparation of alloy materials. In crucible 6 a shaped elongated mass of-germanium 2', for example 300 grams in a tapered bar 30 cm. long and 1.3 cm. square in maximum cross section, which is the major ingredient of an alloy prepared according to the present invention, is introduced into the boat-type Crucible. A 27 cm. long bar 17 of silicon with a 3 cm. long wedge-shaped end 16 which weighs 8.9 grams is disposed beneath the germanium charge and is contiguous linearly to the germanium charge. The dimensions of the non wedge-shaped section preferably are .3 x .5 cm. x 24 cm. long. The end 16 of the silicon toward the seed is tapered to provide a Oto 10 mol percent gradual increase in silicon concentration and the remainder is of uniform rectilinear dimensions to provide a uniform 10 mol percent silicon composition. A crucible charged in this way and including a single crystal germanium seed can be zone-melted at a rate of 0.5 cm. per hour or less to yield uniformly 10% silicon single crystalli'ne alloy containing 10 grams of alloy per linear cm. by employing an annular furnace in the same way as described in Example I and illustrated in Figure 4. The crystal product may be of any desired length.

When silicon is the alloying agent or minor component and germanium is the base material or major component, the silicon distribution coe'icient is 3.0. Hence, the concentration of the silicon in molten germanium is 1/3 the concentration of the silicon in crystalline germanium under the conditions of crystallization from a melt.

Example vIII In Figure 5 a pot-type crucible 18 in a container 7 and on a support 9 and heated by conventional means, not shown, is charged with silicon 20, for example 50 grams, and a seed of single crystal silicon 22 attached to a withdrawing apparatus 24 is touched onto the surface of the molten silicon in the Crucible and slowly withdrawn with an elongated single crystal 23 attached thereto. As the seed crystal is withdrawn, at a rate of 0.5 cm. per hour or less, for example, a rod 26 of germanium for example of .5 cm. diameter is gradually pushed into the silicon melt by a feed mechanism 23 where it melts and diffuses throughout the molten mass. The feed is adjusted so that the change in the relative proportion of silicon and germanium in the growing crystal is about l mol percent germanium for each hour of growth. This provides a grown crystal of initially pure silicon with the concentration of germanium gradually increasing along its length until a desired composition is reached and maintained by the feed of fresh material to the melt. A fresh charge of silicon can be added to the crucible in the same way that the alloy agent germanium is added.

Although the examples provided include only germanium, silicon and indium, other ele-ments can also be used in the fabrication of alloys according to the present invention. For example, tin, lead or titanium may be substituted in the given examples for either germanium or silicon. Moreover, base materials such as cadmium telluride and gallium antimonide can be modified and doped or alloyed with agents such as germanium and silicon by any of the methods of the present invention.

The single crystalline product grown according to the present invention is characterized by an `apparent macroscopic uniformity and homogeneity. However, it is probable that, if the product could be examined on an atomic scale and the position of each individual atom in the lattice structure were determined, it would appear that the minor component had a random distributionwith local fluctuations in concentration.

The crystal growing method of the present invention is adaptable to the fabrication of single crystalline germanium-silicon alloy of any percentage germanium con tent from zero to 100%. This complete miscibility of the'se two materials is shown by the phase diagram of Stohr and Klemm, Zeitz. Anorg. und Allgem. Chemie, 241, 305323 (1939).

One of the features of the present invention is directed to the elimination of polycrystalline characteristics of a grown crystal. Polycrystalline structure is often the result of an abrupt change in the lattice spacing ofthe individual atoms in the lattice framework.

A feature of the present invention is the gradual smooth *building up of a seed crystal of a base material into an alloy composition containing a large proportion of alloying agent, for example 50 mol percent. According to one aspect ofthe present invention `a minor component is introduced into the lattice of the major component while the crystal is |growing. By a technique of progressively increasing the alloying agent content of a crystal, it is possible to produce a crystal composition which contains a large proportion of alloying agent, and by continuing to add `alloying agent at the appropriate gradual rate to successive segments a crystal can be formed of a desired proportionate alloy content. v

By avoiding abrupt changes in composition and hence in melting points of adjacent regions of the growing crystal, it is possible to make homogeneous single crystalline alloys in a very Wide range of compositions. Other methods of forming these large single crystals of these alloys are either much more limited in the possible range of compositions, or are very diicult and tedious to perform.

There have thus been described novel methods based upon a principle of gradual alloying for the fabrication of uniform homogeneous single crystalline semiconductor materials.

What is claimed is:

1. The method of manufacturing single crystal alloy material which comprises in combination the steps of providing a mass consisting essentially of one constituent of said alloy material, contacting a portion of said mass with a seed crystal made of the same substance as said one constituent, melting said portion of said mass which is in contact with said seed crystal and also melting the remainder of said mass, solidifying said portion to start a single crystal growing, applying to said mass in its molten form successively increasing predetermined quantities of another constituent of said alloy material in the solid state thereof, said quantities being successively increased until sufficient to establish a predetermined constant concentration ratio of said alloy constituents ultimately desired, and melting successive quantities of said other constituent of said alloy material to cause both of said constituents to alloy and also successively solidifying said mass and said other constituent to form a single crystal of said alloy, said crystal being non-homogeneous along its length until said predetermined constant concentration ratio is established therein.

2. The method of manufacturing single crystal alloy material which comprises in combination the steps of providing a solid mass consisting essentially of one constituent of said alloy material, applying to successive portions of said mass successively increasing predetermined quantities of another constituent of said alloy material in the solid state thereof, said quantities being successively increased until suicient to establish the predetermined `constant concentration ratio of said alloy constituents said seed crystal and then solidifying said portion to start a single crystal growing 'and successively heating the remainder of said portions of said mass to cause both of said constituents to melt and to allow and also successively solidifying said portions to form a continuous single crystal of said alloy, said crystal being non-'homogeneous along its length until said predetermined constant concentration ratio is established therein.

3. A method of manufacturing a germanium-silicon single crystal alloy material which comprises in combination the steps of providing `a solid mass consisting essentially of one of the constituents of said alloy material, 'applying to successive portions of said mass successively increasing predetermined quantities of the other constituent of said alloy material in the solid state thereof, said quantities being successively increased until sucient to establish the predetermined constant concentration ratio of said alloy constituents ultimately desired, contacting a portion of said mass with a seed crystal made of the same substance as said one constituent, melting said portion which is in contact with said seed crystal and then solidifying said portion to start a'hsingle crystal growing and successively heating the remainder of said portions of said mass to cause both of said constituents to melt and to alloy and also successively solidifying said portions to form a continuous single crystal of said alloy, said crystal being non-homogeneous along its length until said predetermined constant concentration ratio is established therein.

4. The method of claim l wherein substantially all of said mass of said one constituent is in molten form when contacted with said seed crystal and said solidiication steps are accomplished by slowly withdrawing said seed crystal from the molten mass.

5. The method of claim 4 wherein said single crystal alloy material comprises an alloy of germanium and silicon.

References Cited in the le of this patent UNITED STATES PATENTS 935,358 Davis Sept. 28, 1909 20 2,175,606 Kinkead Oct. 10, 1939 2,623,105 Shockley etal Dec. 23, 1952 2,739,088 Phann Mar. 20, 1956 

1. THE METHOD OF MANUFACTURING SINGLY CRYSTAL ALLOY MATERIAL WHICH COMPRISES IN COMBINATION THE STEPS OF PROVIDING A MASS CONSISTING ESSENTIALLY OF ONE CONSTITUENT OF SAID ALLOY MATERIAL, CONTACTING A PORTION OF SAID MASS WITH A SEED CRYSTAL MADE OF THE SAME SUBSTANCE AS SAID OEN CONSTITUENT, MELTING SAID PORTION OF SAID MASS WHICH IS IN CONTACT WITH SID SEED CRYSTAL AND ALSO MELTING THE REMAINDER OF SAID MASS, SOLIDIFYING SAID PORTION TO START A SINGLE CRYSTAL GROWING, APPLYING TO SAID MASS IN ITS MOLTEN FORM SUCCESSIVELY INCREASING PREDETERMINED QUANTITIES OF ANOTHER CONSTITUENT OF SID ALLOY MATERIAL IN THE SOLID STATE THEREOF, SAID QUANTITIES BEING SUCCESSIVELY INCREASED UNTIL SUFFICIENT TO ESTABLISH A PREDETERMINED CONSTANT CONCENTRATION RATIO OF SAID ALLOY CONSTITUENTS ULTIMATELY DESIRED, AND MELTING SUCCESSIVE QUANTITIES OF SAID OTHER CONSTITUENTS OF SAID ALLOY MATERIAL TO CAUSE BOTH OF SAID CONSTITUENTS TO ALLOY AND ALSO SUCCESSIVELY SOLIDIFYING SAID MASS AND SAID OTHER CONSITUENT TO FORM A SINGLE CRYSTAL OF SAID ALLOY, SAID CRYSTAL BEING NON-HOMOGENEOUS ALONG ITS LENGTH UNTIL SAID PREDETERMINED CONSTANT CONCENTRATION RATIO IS ESTABLISHED THEREIN. 